1. Field of the Invention
The present invention relates to the manufacture of contact lenses via cast molding, and more particularly to the cast molding of contact lenses having contoured edges utilizing an improved mold with anti-tilt features to improve the lens edge around the full circumference of the lens.
2. Discussion of the Related Art
Historically, contact lenses were manufactured by machining a lens material, in button or block form, to produce an unfinished lens product having the required fit or base curve and visual correction to compensate for one or more refractive abnormalities or errors of the eye. Such refractive abnormalities or errors include myopia, hyperopia, astigmatism and presbyopia. Utilizing conventional machining technology, for example, a lathe, the optic faces of the unfinished lenses required polishing in order to remove turning rings produced in the machining process. Typically, the edge of the machined lenses was also polished to produce a smooth edge geometry and/or profile in order to maximize wearer comfort and prevent injury to the eye.
The machining and polishing process described above is labor intensive and has been widely replaced by cast molding. Cast molding of contact lenses offers a number of advantages over machined lenses. For example, with a cast molding process it is possible to produce contact lenses with a high level or degree of repeatability. In other words, with a cast molding process it is possible to produce many lenses which have the same shape and optical characteristics. In addition, a molded contact lens may be formed to any desired shape, subject only to the producibility constraints of the mold.
In general, molded lenses are formed by depositing a curable liquid such as a polymerizable monomer into a mold cavity, curing the liquid into a solid state, opening the mold cavity and removing the lens. Other processing steps, including hydration, may also be performed. In conventional cast molding techniques, front and back surface mold halves are made for the required lens prescription. Each mold section includes a negative impression of or a contour which corresponds to the desired lens front or back surface. The two mold halves are then assembled and lens material, for example a curable liquid polymerizable monomer, is introduced into the cast mold. After the cast molding steps, subsequent processing steps are employed which may include curing, demolding, hydration in the case of hydrogel lenses, and edge polishing if needed. Fully cast molded lenses are lenses in which all surfaces, front, back and the edge of the lens are molded as part of a mold pair and no post processing polishing or machining operations are performed to yield the final lens product.
Although fully molded lenses provide an inexpensive and flexible alternative to machined and polished cast molded lenses, fully molded lenses are sometimes uncomfortable for the wearer, particularly after long term use. Therefore, it would be advantageous to have a fully cast molded contact lens comprising a desired edge geometry without the need for post processing polishing steps. Currently utilized contact lens molds do not permit production of lens edges that are contoured and which would provide comfort as well as increased tear movement between the lens back or concave surface and the lens wearer's cornea. Accordingly, it would be advantageous to have a lens mold for production of a lens having an edge contoured so as to avoid impingement on the lens' wearer's conjunctional membrane. Additionally, known contact lens molds may be problematic in that they permit movement of one mold half of a mold assembly in relation to the other molds half. This movement results in the lens edge shape varying from one side of the contact lens to the other. Accordingly, it would also be desirable to have a mold design which provides for this consistent edge around the entire circumference of the lens while offering a high degree of repeatability in the manufacturing process.